Conventionally, a battery pack containing secondary batteries has been used as a power source of a handheld electric power tool. A charging circuit for charging the secondary batteries is formed by attaching the battery pack to a dedicated battery charger. In the charging circuit, the secondary batteries are charged with electric power supplied from the outside. The charging circuit for charging the secondary batteries includes a charging unit for rectifying the electric power supplied from the outside and boosting the voltage, a control unit for performing the charging control of the charging unit, a voltage monitoring unit for monitoring the charged voltage of the secondary batteries and a storage unit storing charging data for use in the charging control and the voltage monitoring. The charging data includes a charging setting for the charging control such as an amount of an electric current supplied during a charging operation, a voltage value available during a constant voltage charging process, etc.; and a control voltage value, i.e., an upper limit of a charged voltage, to determine whether the secondary batteries are fully charged.
In keeping with the diversification of the kind of the secondary batteries and the charging capacity of the battery pack, the control unit detects a configuration setting, e.g., the kind of the secondary batteries, from the battery pack attached to the battery charger and performs the charging control by using the charging data corresponding to the respective configuration setting thus detected. In view of this, the battery pack is provided with a storage unit which stores the charging data, and the control unit performs the charging control by reading the charging data stored in the storage unit. There is also known a battery pack which stores plural kinds of charging data in its storage unit to enable appropriate charging data to be used adaptably depending on the kind of the battery charger or the number of charging times.
During the charging operation, the charged voltage monitoring unit detects the electric charges accumulated in a secondary battery from the output voltage thereof, and outputs an electric signal to the control unit if the potential (i.e., charged voltage) of the secondary battery reaches a control voltage. The control unit 2 determines, upon receiving the electric signal, that the secondary battery is fully charged and terminates the charging operation.
Further, there is a demand to shorten the charging time of the secondary battery. The charging time can be shortened by performing such a charging control as to supply a large amount of electric current immediately after starting a charging operation.
In case of supplying the large amount of electric current immediately after starting the charging operation, however, there may be the case that, if a battery pack in a full charge condition or in a near-full-charge condition is attached to a battery charger, the large amount of electric current is supplied to the secondary batteries of the battery pack prior to detection of the full charge condition, eventually overcharging the secondary batteries.
Further, an electric power tool employing a battery pack as its power source is often used in high-load works such as boring or the like. Since the battery pack is caused to discharge a large amount of electric current during the high-load works, there may be the case that the secondary batteries of the battery pack are overheated due to such discharge and comes into a high temperature state. When charging a secondary battery kept at high temperature, a rated amount of electric charges may not be normally accumulated in the secondary battery. Therefore, such a secondary battery kept at the high temperature may be overcharged if it accumulates the same amount of electric charges as in the secondary battery kept at a normal temperature without being overheated.
If the secondary battery is overcharged as mentioned above, the battery cell of the secondary battery is degraded, which reduces total charging capacity of the secondary battery. Therefore, the secondary battery may be overcharged and degraded even when the same amount of electric charges as that properly charged before the occurrence of overcharging is accumulated in the secondary battery. Degradation of the battery cell may lead to not only reduction of the total charging capacity but also easier generation of heat in the discharging and charging operations. In addition, the degradation of the battery cell may cause damage or fluid leakage to the secondary battery, thereby shortening the battery life span. In particular, the overcharging-caused degradation of battery cell appears more remarkably in a lithium-ion secondary battery than in a nickel-hydrogen secondary battery.
Taking this into account, there is known charging circuit of the type in which a safety control for preventing overcharging of a secondary battery is performed by reducing the amount of an electric current supplied to a battery pack immediately after starting a charging operation (that is, at an initial stage of a charging operation) or by reducing a control voltage used as a criterion for determination of charging completion according to the number of times of overcharged states experienced by the battery pack.
For example, Japanese Patent Application Publication No. 2007-143279 discloses a charging circuit in which a safety control is performed by reducing the amount of an electric current supplied to a battery pack at an initial stage of a charging operation. In this charging circuit, extension of the charging time and overcharging of secondary batteries immediately after starting a charging operation are prevented by reducing the amount of an electric current supplied to a battery pack at an initial stage of a charging operation and by increasing the amount of an electric current supplied to the battery pack as the charging time lapses.
Further, as another example, Japanese Patent Application Publication No. 2007-325324 discloses a charging circuit in which a safety control is performed by reducing a control voltage according to the number of times of overcharged states experienced by a battery pack. In this charging circuit, the number of occurrences of overcharged and overheated states experienced during charging operations is stored in a battery pack, and the degree of degradation of secondary batteries is obtained based on the number of occurrences by a charging control determination unit in the battery pack. Thereafter, a charging control unit reduces the control voltage, i.e., potential of the secondary batteries at the completion of charging operation based on the obtained result to prevent overcharging of the degraded battery pack.
In addition, there is known a charging circuit of the type in which a temperature detection unit for detecting the temperature of a secondary battery is provided in a battery pack. If the temperature detected by the temperature detection unit is equal to or higher than a specified temperature (if the secondary battery is in a high temperature state), charging is started after the battery pack is cooled by a cooling unit. In this way, overcharging is prevented by inhibiting the battery pack from being charged in an overheated state.
However, the charging circuit disclosed in Japanese Patent Application Publication No. 2007-143279 is designed to only prevent overcharging immediately after starting a charging operation and may perform overcharging with respect to a battery pack having a degraded battery cell or a battery pack attached to a battery charger in a high temperature state. The charging circuit disclosed in Japanese Patent Application Publication No. 2007-325324 is capable of preventing overcharging of a battery pack having a degraded battery cell but may perform overcharging with respect to a battery pack attached to a battery charger in a high temperature state. In the charging circuit of the type in which charging is started after cooling the battery pack, overcharging may be performed with respect to a degraded battery pack. Moreover, the charging time is prolonged due to the addition of the cooling time.
As discussed above, there exists no charging circuit capable of performing a charging operation according to the presence or absence of battery cell degradation and the change in the temperature of a secondary battery, both of which are causes of overcharging of a battery pack. In particular, there exists no charging circuit capable of performing a charging control according to the temperature change of a secondary battery without prolonging the charging time. Thus, the conventional charging circuits suffer from a problem in that they are inconvenient to use. In the battery pack for electric power tools, overcharging may occur because the charging data stored in the storage unit cannot cope with the case where the secondary battery is considerably degraded by repeating a high-load work and a job-site charging task or the case where charging is performed under a peculiar environment, e.g., within a freezing compartment as a job site in which the temperature of the secondary battery becomes lower than the normal (room) temperature.